An investigation into the role of microstructure in determining strength properties of hydroxyapatite ceramics has been carried out. The investigation consists of two parts. In the first part, monophase hydroxyapatite ceramics has been prepared and characterized. The results obtained from the indentation technique indicate that the monophase hydroxyapatite ceramics has a single-value fracture toughness with the value of 0.9 ± 0.1 MPam½ and is highly susceptible to subsequent in-service strength degradations and wears. In the second part, tetragonal zirconia containing 3 mol% yttria has been uniformly dispersed into hydroxyapatite, using the volume ratio of 20:80. The results obtained from the indentation technique indicate that the incorporation of yttria zirconia improves strength and weare properties of hydroxyapatite ceramics. The prepared zirconia-hydroxyapatite composite shows a much greater resistance to wear than the monophase hydroxyapatite ceramics. Its strength are also found to be significantly higher in the large-crack size domain, although this is offset by lower strengths in the small-crack size domain. Moreover, its strengths are independent of crack size. Such crack-insensitive strength is a desirable property in the context of in-service reliability, because a single, well-defined design stress level can be made without regard to the size of a crack.